(1) Field of the Invention
The present invention relates to a method of producing honeycomb structural bodies consisting of barium titanate series ceramics having a positive temperature coefficient of electric resistance (hereinafter, ceramics having a positive temperature coefficient of electric resistance is referred to as PTC ceramics).
In the remainder of the specification, the term "honeycomb structural body" shall be understood to mean a structural body having a multiplicity of parallel channels extending therethrough, the channels are separated by partition walls which are thin and substantial same in thickness, and having a surface-to-volume ratio in the range of 8 to 40 cm.sup.2 /cm.sup.3.
(2) Description of the Prior Art
There has been disclosed a method of producing a ceramic honeycomb structural body consisting of an insulating ceramic material, for example, cordierite, mullite, alumina and the like, in U.S. Pat. No. 3,824,196 (British Pat. No. 1,385,907) granted on July 16, 1974 to Benbow et al. Further, U.S. Pat. No. 3,927,300 (British Pat. No. 1,427,676) granted on Dec. 16, 1975 to Wada et al discloses a honeycomb structural body consisting of barium titanate series PTC ceramics. However, even when the production method of honeycomb structural body consisting of an insulating ceramic material is merely applied to the production of honeycomb structural bodies consisting of barium titanate series composition, the resulting honeycomb structural bodies do not always have uniform properties.
For example, in the production of cordierite ceramic honeycomb structural body to be used as a catalyst carrier or heat exchanger, raw material can be partly replaced by several to ten several percents of clay in order to give plasticity and good formability to the green mass for forming and to maintain the strength of the formed article from forming to firing and from burning out of the organic binder at the firing to starting of sintering.
However, barium titanate series PTC ceramics consist mainly of BaTiO.sub.3, Ba.sub.1-x Pb.sub.x TiO.sub.3 or Ba.sub.1-y Sr.sub.y TiO.sub.3, which is formed by substituting lead and/or strontium for Ba in BaTiO.sub.3, or BaTi.sub.1-z Sn.sub.z O.sub.3, which is formed by substituting tin for Ti in BaTiO.sub.3, or combinations thereof. Therefore, when a barium titanate series PTC ceramics contain a large amount of SiO.sub.2 and/or Al.sub.2 O.sub.3 which are main ingredients of a clay, the ceramics lose their semiconductivity and positive temperature coefficient of electric resistance, and when the amount of SiO.sub.2 and/or Al.sub.2 O.sub.3 is excessively large, the ceramics become an insulating body, and ceramics having desired electrical properties cannot be obtained. Accordingly, although the use of a raw material containing not less than several percents of clay can obtain a good formed article or a green mass having a good shape-retention, the raw material cannot be used as a raw material for barium titanate series honeycomb structural body.
An alumina or cordierite honeycomb structural body is a porous body having a water absorption of not less than 20%. That is, alumina or cordierite, after it is formed into honeycomb shape and dried, does not noticeably shrink. Therefore, the honeycomb-shaped formed article can be fired without so much caution. On the contrary, the honeycomb structural body consisting of barium titanate series PTC ceramics must be a high density substance having a water absorption of not higher than 1.0% in order to give a good electric semiconductivity to the ceramics and to make the durability as a heating element high. The barium titanate series ceramic materials shrink more than 50% volume during the firing. Therefore, even when the production method of cordierite or alumina honeycomb structural bodies is merely applied to the production of barium titanate series honeycomb structural body, honeycomb structural bodies having good properties cannot be obtained.
Further, a cordierite or alumina honeycomb structural body, even when it is incorporated with a small amount of iron or alkali from raw material or during the production step of the body, can be used without any trouble, but a honeycomb structural body consisting of barium titanate series PTC ceramics, even when it is incorporated with only several tens ppm of transition metal, such as iron or the like, has a high electric resistance and cannot be used for practical purposes.